Precision optical elements require highly polished surfaces of exacting figure and surface quality. The surfaces demand fabrication in proper geometric relation to each other and, where the elements are to used in transmission applications, they will be prepared from a material of controlled, uniform, and isotropic refractive index.
Precision optical elements of glass are customarily produced via one of two complex, multi-step processes. In the first, a glass batch is melted in a conventional manner and the melt formed into a glass body having a controlled and homogeneous refractive index. Thereafter, the body may be reformed utilizing well-known repressing techniques to yield a shape approximating the desired final article. The surface figure and finish of the body at this stage of production, however, are not adequate for image forming optics. The rough article is fine annealed to develop the proper refractive index and the surface figure improved via conventional grinding practices. In the second method the glass melt is formed into a bulk body which is immediately fine annealed and subsequently cut and ground to articles of a desired configuration.
Both processes are subject to similar limitations. The surface profiles that are produced through grinding are normally restricted to conic sections, such as flats, spheres, and parabolas. Other shapes and, in particular, general aspheric surfaces are difficult to grind. In both processes, the ground optical surfaces are polished employing conventional, but complicated, polishing techniques which strive to improve surface finish without comprising the surface figure. In the case of aspheric surfaces, this polishing demands highly skilled and expensive hand working. A final finishing operation, viz., edging, is commonly required. Edging insures that the optical and mechanical axes of a spherical lens coincide. Edging, however, does not improve the relationship of misaligned aspheric surfaces, which factor accounts in part for the difficulty experienced in grinding such lenses.
The direct molding of lenses to the finished state could, in principle, eliminate the grinding, polishing, and edging operations, which are especially difficult and time consuming for aspheric lenses. Indeed, molding processes are utilized for fabricating plastic lenses. Nevertheless, existing plastics suitable for optical applications are available in a limited refractive index and dispersion range only. Furthermore, many plastics scratch easily and are prone to the development of yellowing, haze, and birefringence. The use of abrasion-resistant and anti-reflective coatings has not fully solved those failings. Moreover, plastic optical elements are subject to distortion from mechanical forces, humidity, and heat. Both the volume and refractive index of plastics vary substantially with changes in temperature, thereby limiting the temperature interval over which they are useful.
The overall properties of glass render it generally superior to plastic as an optical material. Conventional hot pressing of glass, however, does not provide the exacting surface figures and surface qualities demanded for image forming optics. The presence of chill wrinkles in the surface and surface figure deviations constitute chronic afflictions. As observed above, similar problems can be encountered in conventional repressing techniques.
Various schemes have been devised to correct those problems, such devices frequently involving isothermal pressing, i.e., utilizing heated molds so that the temperature of the glass being molded will be essentially the same as that of the molds, the use of gaseous environments inert to the glass and mold materials during the pressing operation, and/or the use of materials of specifically defined compositions in the construction of the molds.
U.S. Pat. No. 4,481,023-Marechal and Maschmeyer shows and describes an improved mold for precisely pressing a glass preform which has an overall geometry closely similar to the desired final lens. A top and a bottom mold have molding cavities which precisely match the configuration of the final lens. A glass preform is heated to the molding temperature and the mold parts are separately heated. The molds are brought together against a ring having a thickness which governs the thickness of the lens to be molded.
In such molding operations, the volume of the glass that is put into the molding cavities is controlled by measuring its mass. The density of the glass is accurately known. However, it is desirable to make the mold and the molding process relatively insensitive to the mass of the glass being molded. That is, the same precision shape of the lens should be obtained regardless of minor variations in the mass of the glass put into the mold.
Another requirement for precision molding is that the position of the mold parts be "kinematically determinate" with respect to certain degrees of freedom. A rigid, free body has six degrees of freedom, namely displacements in each of the three orthogonal directions and rotations about each of those same three orthogonal directions. The rigid body's location in space is uniquely described when those three displacements and three rotational angles are defined or fixed. Those three displacements and three angles can be kinematically fixed by constraints. A mechanical contact is such a constraint which will in general fix one degree of freedom. As the contact area between two surfaces becomes small, the contact becomes more ideal in terms of repeatability. A mechanical design is kinematically determinate when the constraints in the system are equal in number to the degrees of freedom associated with the free body. When the degrees of freedom equal the constraints, then there is a single unique position for the free body when it comes into contact with those constraints. See Wilson, Jr. An Introduction To Scientific Research, pp. 104-108, McGraw Hill.
Objects which are rotationally symmetric about one of the axes will have only five degrees of freedom because one cannot distinguish any uniqueness to the object's position about the symmetric axis. Lenses are symmetrical about the optical axis. This removes one degree of freedom, but molds require fixing five degrees of mechanical freedom in order to precisely and repeatedly place them in the correct location to mold a highly accurate lens.
It is an object of the present invention to provide an improved glass mold and molding operation which precisely sets the final mold positions to precisely form the desired lens configuration.